1. Field of the Invention
The present invention relates to a dibenzo[f,h]quinoxaline compound, a light-emitting element, a light-emitting device, an electronic device, and a lighting device.
2. Description of the Related Art
In recent years, research and development have been extensively conducted on light-emitting elements utilizing electroluminescence (EL). In the basic structure of such a light-emitting element, a layer containing a light-emitting substance is interposed between a pair of electrodes. By voltage application to this element, light emission from the light-emitting substance can be obtained.
Such light-emitting elements are self-luminous elements and have advantages over liquid crystal displays in having high pixel visibility and eliminating the need for backlights, for example; thus, such light-emitting elements are thought to be suitable for flat panel display elements. Such light-emitting elements are also highly advantageous in that they can be thin and lightweight. Furthermore, very high speed response is one of the features of such elements.
Since light-emitting layers of such light-emitting elements can be formed in a film form, they make it possible to provide planar light emission. This is a feature difficult to obtain with point light sources typified by incandescent lamps and LEDs or linear light sources typified by fluorescent lamps. Thus, the light-emitting elements also have great potential as planar light sources applicable to lightings and the like.
Light-emitting elements utilizing electroluminescence can be broadly classified according to whether a light-emitting substance is an organic compound or an inorganic compound. In the case of an organic EL element in which a layer containing an organic compound used as a light-emitting substance is provided between a pair of electrodes, application of a voltage between the pair of electrodes causes injection of electrons from a cathode and holes from an anode into the layer containing the organic compound having a light-emitting property and thus a current flows. Recombination of the injected electrons and holes then leads the organic compound having a light-emitting property to its excited state, whereby light emission is obtained from the excited organic compound having a light-emitting property.
The excited state formed by an organic compound can be a singlet excited state or a triplet excited state. Emission from the singlet excited state (S*) is called fluorescence, and emission from the triplet excited state (T*) is called phosphorescence. In addition, the statistical generation ratio thereof in a light-emitting element is considered to be as follows: S*:T*=1:3.
In a compound that emits light from the singlet excited state (hereinafter, referred to as a fluorescent compound), at room temperature, light emission from the triplet excited state (phosphorescence) is not observed while only light emission from the singlet excited state (fluorescence) is observed. Therefore, the internal quantum efficiency (the ratio of generated photons to injected carriers) of a light-emitting element using a fluorescent compound is assumed to have a theoretical limit of 25% based on the ratio of S* to T* which is 1:3.
In contrast, in a compound that emits light from the triplet excited state (hereinafter, referred to as a phosphorescent compound), light emission from the triplet excited state (phosphorescence) is observed. Further, in a phosphorescent compound, since intersystem crossing (i.e., transfer from a singlet excited state to a triplet excited state) easily occurs, the internal quantum efficiency can be increased to 100% in theory. In other words, a light-emitting element using a phosphorescent compound can easily have higher emission efficiency than a light-emitting element using a fluorescent compound. For this reason, light-emitting elements using phosphorescent compounds are now under active development in order to realize highly efficient light-emitting elements.
When a light-emitting layer of a light-emitting element is fowled using a phosphorescent compound described above, in order to suppress concentration quenching or quenching due to triplet-triplet annihilation in the phosphorescent compound, the light-emitting layer is often formed in such a manner that the phosphorescent compound is dispersed in a matrix of another compound. Here, the compound serving as the matrix is called a host material, and the compound dispersed in the matrix, such as the phosphorescent compound, is called a guest material.
When a phosphorescent compound is used as the guest material, one of the properties that the host material needs to have is a triplet level (energy difference between a ground state and a triplet excitation state) higher than that of the phosphorescent compound.
Furthermore, since a singlet level (energy difference between a ground state and a singlet excited state) is generally located higher than a triplet level, a substance that has a high triplet level also has a high singlet level. Therefore, the above substance that has a high triplet level is also effective in a light-emitting element using a fluorescent compound as a light-emitting substance (a guest material).
Studies have been conducted on compounds having dibenzo[f,h]quinoxaline rings, which are examples of the host material used when a phosphorescent compound is a guest material (e.g., see Patent Documents 1 and 2).